By DAVID E. SANGER

Published: September 13, 2004

Senator John Kerry on Sunday accused the Bush administration of letting ''a nuclear nightmare'' develop by refusing to deal with North Korea when it first came to office.

In an interview, he argued that President Bush's preoccupation with Iraq let the North Korean crisis fester to the point that there were now indications that the country might be preparing to test a plutonium bomb. He presented his charges in a 15-minute telephone call he made to The New York Times.

Meanwhile, American intelligence officials and policy makers scrambled to determine what caused a huge fire spotted Thursday on North Korea's border with China. Secretary of State Colin L. Powell said Sunday that the fire, apparently caused by an explosion, was ''not any kind of nuclear event,'' but he confirmed a report in The Times on Sunday that ''there is activity going on at a potential nuclear test site,'' adding that ''we're monitoring this.''

While intelligence analysts are still debating whether the activity is a harbinger of a test, Mr. Kerry insisted that the fact that North Korea was threatening such an action was a sign of failed diplomacy. ''I think that this is one of the most serious failures and challenges to the security of the United States, and it really underscores the way in which George Bush talks the game but doesn't deliver,'' he said.

In the past, Mr. Kerry has accused Mr. Bush of ignoring a far larger nuclear threat in North Korea because of his determination to oust Saddam Hussein in Iraq.

''They have taken their eye off the real ball,'' Mr. Kerry said, his voice almost shaking in anger. ''They took it off in Afghanistan and shifted it to Iraq. They took it off in North Korea and shifted it to Iraq. They took it off in Russia, and the nuclear materials there, and shifted it to Iraq.''

Mr. Kerry's basic argument, that the Iraq war has diverted attention from more dangerous nations like North Korea, is one he has often used on the campaign trail and in interviews over the past several months. But his language on Sunday, calling the situation ''a nuclear nightmare'' and directly accusing Mr. Bush of leaving the United States more vulnerable to North Korea, was far harsher and more incendiary than the language he has used before.

It is also highly unusual for Mr. Kerry to seek out a reporter on a Sunday, when he had no public appearances scheduled, to attack Mr. Bush. This comes as Mr. Kerry and his aides, during this final 50 days of the campaign, have promised to draw more consistent and sharper contrasts to Mr. Bush in response to criticism from supporters that their message has been too weak.

On Sunday night, Scott McClellan, the president's press secretary, told of Mr. Kerry's comments, said: ''Senator Kerry wants to return to the failed policies of the previous administration, where the U.S. was duped. We've been down that road before and we have no intention of letting it happen again.''

In television appearances on Sunday, both Mr. Powell and Condoleezza Rice, Mr. Bush's national security adviser, defended the administration's approach to North Korea. They insisted that they had a far better chance of forestalling a test by North Korea now that the United States had involved China, Russia, Japan and South Korea in negotiations rather than facing North Korea alone.

Ms. Rice said it was possible that the fire seen Thursday -- the South Korean press described it as a mushroom-shaped cloud of smoke that extended two miles across -- might have been a forest fire. American intelligence officials said they were also pursuing the possibility that fuel used for North Korea's missiles might have accidentally exploded.

[North Korea said Monday that the explosion was part of a hydro-electric project, the BBC reported. It quoted the North's foreign minister, Paek Nam Sun, as telling the visiting British Foreign Office minister Bill Rammell that the blast was the deliberate demolition of a mountain for the project.]

But it was the prospect of a nuclear test -- perhaps one timed by the North Koreans before the presidential election here -- that focused the administration's attention.

''It's no longer just North Korea versus the United States,'' Mr. Powell said on the ABC News program ''This Week.'' ''It's North Korea versus all of its neighbors, which have no interest in seeing North Korea with a nuclear weapon.''

Mr. Kerry argued that it was the Bush administration's refusal to follow Mr. Powell's advice in March 2001 and continue the Clinton administration's direct diplomacy with North Korea that created the conditions for the current crisis. Mr. Kerry has said for months that the United States must deal directly with the North Korean government -- just as it dealt directly with Khrushchev during the Cuban missile crisis, and directly with China as it became a nuclear power in the 1960's.

''The week that Colin Powell stood up and said we are going to continue the dialogue with the North Koreans, I said, 'Good,''' Mr. Kerry said, recalling what, in retrospect, was the first major foreign policy split in the administration, months before the Sept. 11 attacks. ''And two days later when George Bush pulled the rug out from under Kim Dae Jung when he was in the Oval Office, and sent him back both embarrassed and bewildered to South Korea, I said that was both dangerous and the wrong direction for this issue and America.''